The use of materials with functionally graded characteristics has recently attracted the interest of engineers due to a new range of solutions in products design. The use of Ni-Ti wire with these properties allows to create a graded distribution of the forces exerted by wires and can be used for instance in orthodontic applications or in actuators.
FIBR3D project aims to develop an additive manufacturing feedstock able to be processed by Fused Deposition Modelling (FDM). The feedstock should be reinforced by an advanced functionally graded material (FGM), the gradient being induced through heat-treatment. For a FGM, the main techniques for identification and characterization of the properties of the heat-treated zone involve destructive testing, such as tensile and torsion tests or differential scanning measurements. Such course of action requires extra time and resources to achieve the desired characteristics for the heat-treated zone. The objectives of the present work are to monitor the heat treatment of a 0.4 mm Ni-Ti wire in real time, detecting changes of the electrical resistivity. The phase changes taking place during in-service heating and cooling cycle will also allow to characterize the different heat-treated zones with a non-contact technique.
Four-point technique was used to measure the electrical resistivity between electrodes during the heat treatment by Joule effect. A customized eddy current probe was developed to characterize wires without contact. Numerical simulation was used to compare ant optimize several eddy current probe designs, allowing to create a dedicated and customized probe. The probe was produced and experimentally validated.
Electrical resistivity measurement of Ni-Ti wires has proven to be a good probe for the identification of various phase transformations (thermally and stress induced). The length and phase of each heat-treated zone were well defined and characterized by eddy current technique.